The "impact zone" is that portion of the golf swing during which the clubhead is in contact with the ball and thus delivering energy to the ball. It is solely responsible for the ball flight.
In order to achieve optimal clubhead alignment during the "impact zone", it is desirable to keep the clubhead square (i.e. perpendicular to the target line) and on a path toward the target during the "impact zone" and for a short distance (one to two feet) subsequent to the "impact zone". The portion of the golf swing from initial contact through this short distance subsequent to the "impact zone" during which the clubhead is square and moving toward the target is commonly referred to as the "hitting zone". In reality there can be no effect on the ball after it has left the club, however, it generally is accepted that focusing on the golf swing through the "hitting zone" will positively influence what happens during the "impact zone".
There have been a number of devices developed which claim to help a player analyze his or her golf swing. Barton, U.S. Pat. No. 4,022,476, discloses a golf practice device with a ball which is adapted to be quickly stabilized after it is hit to allow a second stroke. Wang, U.S. Pat. No. 4,932,660, discloses a golf practice device in which a ball is attached to a rigid arm. When the ball is hit the arm causes a tubular portion of the device to rotate to allow the ball to rotate. D'Allura, U.S. Pat. No. 5,121,923, also discloses a golf training device in which a ball is attached to a rigid rod that allows the ball to rotate. Schafer, U.S. Pat. No. 3,827,696, discloses a golf swing training device with a golf ball suspended by a tether from an attachment bar, and an adjustable sighting guide located between the golfer's head and the tethered golf ball. Oppenheimer, U.S. Pat. No. 3,472,075, discloses a golf simulation system with a ball on the end of a rigid arm which itself is operatively connected to some gearing which indicates whether or not the ball left the club face in a straight path. Moffatt, U.S. Pat. No. 2,929,632, also discloses a golf practice device with a tethered ball rotatably mounted to a horizontal arm. MacDonald, U.S. Pat. No. 1,419,636, discloses a test and practice apparatus for golf players with a target comprising a golf ball mounted at the upper end of a pivoted vertical stem, and a spaced secondary target constituting a rotatable dumbbell or bars, or bars that are not rotatable but that are adapted to be thrown to a horizontal position when struck by the club, in order to indicate the path of the club through the hitting zone.
The devices with a single ball do not accurately reflect the swing through the impact zone. As the club face strikes the ball, because the ball is tethered, it is forced up and away from the club face. Accordingly, the ball is forced to leave the club face long before the club face has left the hitting zone and, in fact, before it has left the impact zone. In actual play, the club compresses the ball on impact and the club face stays in contact with the ball for a period of time after initial contact. The time during which the club face is in contact with the ball is the impact zone. The path and orientation of the club face throughout the impact zone is what actually determines the flight of the golf ball. In fact, the most important club position is at the end of the impact zone when the ball actually leaves the club face: this position is primarily what dictates the ball's flight tendencies. And, the single ball devices only indicate the club face orientation at the very beginning of the impact zone, a poor indicator of the actual swing.
By analyzing the clubhead orientation and path throughout the "hitting zone" one can more accurately ascertain the effect of forces applied to the ball during the "impact zone ".
MacDonald has two or more targets which give the player an idea of the hitting zone distance and the path of the club head through the hitting zone. However, the first target does not rotate, it is simply knocked over. Accordingly, the first target gives no indication of the club velocity or direction at the beginning of the hitting zone. Although the secondary target or targets in two embodiments of the MacDonald device do rotate, their speed of rotation is irrelevant as they cannot be compared to the speed of rotation of the first target, since it does not rotate. Accordingly, the MacDonald device does not provide feedback on both the quality of the club face orientation as well as the acceleration of the club throughout the hitting zone.